1. Field
The present invention generally relates to operations performed by a wireless mobile station to preserve communications with a central facility despite movement among coverage areas. More particularly, the invention concerns a wireless subscriber station, and method for its operation, where the mobile station manages the timing and selective performance of “off-frequency searches,” that is, searches for base station pilot signals that differ in frequency from the pilot signals of the subscriber station's active set.
2. Background
Mobile phone designers are faced with a variety of different engineering challenges. One of the most perplexing problems is the necessity of using battery power to drive the phone's transceiver, speaker, microphone, display, and all other on-board electronics. A battery can only provide a finite amount of power until exhaustion, at which time the phone ceases to work. Of course, most mobile phone batteries are rechargeable, but this requires access to a power source.
Consequently, mobile phones have been designed with the ability to operate in various low power modes. With the absence of any outgoing or incoming calls, a mobile phone is in an “idle” state. At times, some phone models enter a “sleep” mode where the phone selectively disables various circuitry such as its transceiver, central processor, and certain other hardware. At this point, the phone consumes hardly any current. At periodic intervals that are dictated by the network, the phone briefly awakens, chiefly to receive paging signals from base stations alerting the phone to incoming calls and for other reasons such as searching for pilot signals of nearby base stations, etc. When the mobile phone cannot achieve communications with any base stations for some period of time, the phone enters a “deep sleep” mode, during which the phone waits for user input and meanwhile powers-down to an even greater extent.
Designers of wireless communications systems are continually seeking to further reduce the power consumption of mobile phones. One area of possible focus concerns the periodic wakeup schedule, and more particularly, the search for nearby base stations.
The power consumed during this search is even greater when any nearby base stations utilize pilot signals of a different frequency (“off-frequency”) than the base stations that the mobile is presently monitoring (referred to as the mobile's “active set”). In this case, extra power is consumed due to the necessity for the mobile to adjust its transceiver to each off-frequency. Theoretically, the mobile may be programmed to limit its search to on-frequency base station pilot signals, simply omitting the search for any off-frequency base stations. However, this exposes the mobile to the possible danger of losing coverage by unknowingly traveling into the area of an off-frequency base station and then proceeding sufficiently further so as to completely lose coverage from earlier on-frequency base stations without having established communications with the new, off-frequency base station. Then, the mobile could not initiate or receive telephone calls without performing a time and power consumptive reinitialization process. Accordingly, progress remains to be made in the area of periodic wakeup, and particularly, the search for off-frequency base stations.